water edging plants

We are lucky enough to have a garden which has a small stream running through it.  This never gets more than 1 - 2inches deep and is continually flowing.

It is bordered by 8ft high banks which have been planted in the past with various shrubs heathers ferns, but in places creeping buttercup and ground elder try to rule.  We do not really have any water edging plants down there and I would really like some.  This year in spring I split some hostas and put those there and they have done really well about 1 foot away from the water.  I intend to split more hostas in the autumn as they are getting ratehr large in the rest of the garden and free plants are the best!

Could I get some recommendations for other edging plants that will beat the buttercup and ground elder though I do my best to dig these out.  the ground elder not so much a problem near the water but the buttercup loves it. We are also on a tight budget so nothing too expensive!

The stream is only a foot wide, so nothing too tall either. We live in Scottish borders and it is COLD here in the winter, though this is a wooded area and so does not get frost too much.

thanks

Hemerocallis associate well with hostas and like life near water.  They're not expensive and soon bulk up so can be divided to create more plants.  Astilbes also like moist soil.  I have a purple/lilac flowered one which is spreading very happily ina damp bed and have recently planted some white forms near our own unlined pond.

Forms of gunnera (not all as huge as mannicata) like damp soil as do eupatorium, lysimachia Firecracker and Vesuvius, hydrangeas, miscanthus zebrinus, assorted irises (check for marginal pond varieties), iris sibirica, forms of salix with colourful stems that you cut back in spring to keep the stem colour, rodgersias, acteas, aruncus, dicentras and so on.

I think we have Hemerocallis in the garden but I didn't know that was what they were called (looked it up on google), we also have astilbes so I could divide them as well - oh happy days!

I would love a giant gunnera mannicata - my neighbour has one so I know it will grow here, but I have never got round to getting one at the right time of year, but I will look into smaller varieities as well - thanks very much for the suggestions.

Obelixx has said everything I was going to say, with the exception of daffodils - the original British native Narcissus pseudonarcissus would look lovely there in the spring 

What about Solomon's Seal?